1. Field of the Invention
This invention relates to the fabrication of thin films used in integrated circuits, more specifically to fabrication of materials used in high dielectric constant capacitors for integrated circuits.
2. Statement of the Problem
Metal oxide materials, such as barium strontium titanate ("BST"), have become important for making integrated circuit thin film capacitors having high dielectric constants. Such capacitors are useful in fabricating integrated circuit memories such as DRAMs. See for example, Kuniaki Koyama et al., "Stacked Capacitor with (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IDEM (International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. The capacitors used in a DRAM integrated circuit are the predominant element determining the size of each DRAM cell. To reduce the DRAM cell size, and thereby increase DRAM cell densities in an integrated circuit, requires reduction in the size of the capacitor. Reducing the capacitor size is achieved by increasing the dielectric constant of the material used in the dielectric layer of the capacitor so that a smaller surface area is required for a capacitor having the desired dielectric properties. Prior methods for increasing the dielectric constant of material have also increased the leakage current of the material. Excessive leakage current renders the material unfit for capacitors in integrated circuits and in particular, unfit for capacitors in DRAM cells. It remains a problem in the field to increase the dielectric constant of materials, even for high dielectric constant material such as BST, without significantly increasing the leakage current.
3. Solution to the Problem
The present invention improves upon prior fabrication methods for BST to increase the dielectric constant of the dielectric material for use in integrated circuit memories, such as DRAMs, by blending excess B-site materials with the barium, strontium, and titanium liquid precursors used to produce the dielectric layer. The excess B-site material increases the real part of the dielectric constant of the dielectric layer of the capacitor with little or no increase in leakage current.
The process of the present invention includes blending of excess B-site material such as titanium with liquid precursors comprising barium, strontium, and titanium to form a homogenous liquid suitable for spin-on deposition processes. The liquid precursor preferably includes the B-site material as a substituent to an organic complex. Liquid precursors suitable for this process are preferably metal carboxylates or metal alkoxides. Co-pending U.S. patent application Ser. No. 08/132,744 filed Oct. 6, 1993, recites the use of an alkoxycarboxylate liquid precursor in the fabrication of BST. Co-pending U.S. patent application Ser. No. 08/165,082 filed Dec. 10, 1993, which is hereby incorporated by reference, recites the use of a spin-on process with liquid precursors to form a layer BST. Applying similar "spin-on" methods for fabrication to the present invention permits more accurate control of the structure and distribution of the excess B-site material within the BST dielectric layer of integrated circuit capacitors.
The quantity of excess B-site material preferably ranges from an amount greater than zero (i.e., about 0.01 mole percent) to 100 mole percent ("mol %") of the stoichiometric amount that is required to satisfy the general formula ABO.sub.3, wherein a 1:1:3 molar ratio exists between A (an A-site material), B (a B-site material), and oxygen. Accordingly, the total amount of B-site material preferably ranges from about 100.01 to about 200% of the stoichiometric amount from the general formula. This excess B-site material more preferably ranges from about 0.1 to 20 mol %, and most preferably ranges from about 1 to 3 mol % of the stoichiometric amount.
The precursor solution including excess B-site material is applied onto a substrate preferably using a spin-on process to form a thin film. The coated substrate is then heated preferably within the range of 200.degree. C. to 500.degree. C. to remove the organic residue from the applied thin film. The thin film is then annealed preferably at a temperature ranging from about 600.degree. C. to 850.degree. C. in the presence of oxygen. These methods result in fabrication of high quality BST with excess B-site material exhibiting improved dielectric properties for use in integrated circuit capacitors such as DRAMs. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.